I think it is a bit awkward for such a large room. As mentioned, you could fit 2 single beds comfortably, but probably under the windows, end to end. A double would fit but only realistically if one side is pushed against the wall, which is a significant compromise. I think you should definately not have the built-in cupboard, as it restricts your options. Are the dimensions fixed, e.g. are you building above something else? The radiator would be best behind the door as this is dead wall space anyway.

It’s to the centre of the head, so will be pretty much in the middle of the shower tray if fitted to the side wall. Surely that’s where you want it?

Size of the planks is really up to you, small or large can look great even in small rooms, as for direction you'll want to lay them in a direction that feel right for the room, traditionaly its oblique to where the joists would run. So for the large living room they would run up and down the length of the room (4.371 direction) that would make the room feel longer and be conventional, though you could run the along the shorter length which could make the room feel shorter but cozier.

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